1. Field of the Invention
The present invention relates to a method of manufacturing a wiring board, a photomask, a wiring board, a circuit element and a communications device.
2. Description of the Related Art
A structure having the interior hermetically sealed by using sealed glass has been conventionally known. Among said structures, large ones include a cathode ray tube or a plasma display panel, for instance, and small ones include an SAW device or a crystal device.
In addition, in recent years, high-frequency signals in GHz band for carrying out high-speed and large volume communications have come into use. For this reason, need for a high frequency device (hereinafter referred to as an MEMS device) by MEMS (Micro Electro Mechanical System) technique has increased. For instance, MEMS devices include such devices as a switch, a relay (switching element), a filter, a resonator, a phase shifter, etc. In such the MEMS devices, as it is necessary to protect a mechanism (an operating unit or a contact unit, etc.) of the device against corrosion, dust, etc., and to improve responsiveness by depressurizing an internal space containing the mechanism, the space including the mechanism is sealed by said sealed glass.
A structure for transmitting a high frequency signal includes a structure of microstrip line type or coplanar type. In particular, in the structure of coplanar type, as downsizing is easy to plan and electrical bonding strength between wires is weak, the coplanar type structure is often used in MEMS devices.
Now we describe an MEMS device having the coplanar type structure (hereinafter referred to as a MEMS switching element) that functions as a switching element (relay). As shown in FIG. 22, a MEMS switching element comprises a wiring board 91, a movable electrode 92, fritted glass (sealed glass) 93, and a cap 94. In addition, the wiring board 91 comprises a glass substrate 91a, signal lines 91b, 91c, a fixed electrode (GND) 91d, fixed contacts 91e, 91f, and bonding pads 91g . . . In addition, the signal lines 91b, 91c, the fixed electrode 91d, the fixed contacts 91e, 91f and the bonding pads 91g . . . are formed on the glass substrate 91a. Furthermore, the movable electrode 92 comprises a movable contact 92a. In addition, the movable electrode 92 is biased toward the cap 94 by a spring (not shown).
The MEMS switching element 90 also has a configuration that can apply voltage between the fixed electrode 91d and the movable electrode 92 through the bonding pads 91g . . . Then, electrostatic force generated by the voltage attracts the movable electrode 92 to the side of the wiring board 91, thereby bringing the movable contact 92a into contact with the fixed contacts 91e, 91f. With this, the signal line 91b is electrically connected with the signal line 91c. On the one hand, stopping the application of said voltage releases the connection between said signal line 91b and the signal line 91c (i.e., they are isolated). Thus, switching ON/OFF of the switch is implemented by applying or not applying said voltage.
In the MEMS switching element 90, the wiring board 91 is bonded with the cap 94 by heating and melting the fritted glass 93 that has been bonded to the cap 94 in advance, and applying predetermined pressure between the cap 94 and the wiring board 91. In addition, the surface of the wiring board 91 is odd-shaped (i.e., a shape having steps) by the signal lines 91b, 91c, and the bonding pads 91d, . . . , as shown in FIG. 23. Thus, in the area where the fritted glass 93 contacts the wiring board 91 (hereinafter referred to as a junction area (See FIG. 22.)), as shown in FIG. 24, the fritted glass 93 enters a gap G1 between the signal line 91b and the bonding pad 91g and a gap G2 between the bonding pads 91g. Then, entry of the fritted glass 93 into said gaps (G1, G2) results in hermetic sealing of a space containing the movable electrode 92.
Now we describe a method of manufacturing a MEMS switching element 90.
First, a metal thin film for the signal lines 91b, 91c, the fixed electrode 91d, and the bonding pads 91g . . . is formed on glass wafer. Then, a pattern by a resist (hereinafter referred to as a resist pattern) is generated on the metal thin film, by using a photomask on which a predetermined pattern is formed. Furthermore, etching is performed with this resist pattern as a mask, to selectively remove the metal thin film. Then, the resist is removed. Further, an insulation protective film is formed on the fixed electrode 91d. Thus, a fixed substrate comprised of a plurality of wiring boards 91 has been formed.
Then, after the respective movable electrodes 92 are generated at predetermined positions on the respective wiring boards 91, under reduced pressure, a substrate formed of a plurality of contiguous caps 94 (hereinafter referred to as a glass cap substrate) is joined to the fixed substrate so as to cover the respective movable electrodes 92.
Then, the fixed substrate to which the glass cap substrate is joined is cut (i.e., subjected to dicing) into discrete chips (MEMS switching elements), together with the glass cap substrate. Thus, a plurality of MEMS switching elements 90 are produced at one time, by using so-called wafer level packaging technology (technology of packaging chips prior to dividing them into individual chips). In addition, as the above etching, in general, wet etching is used.
However, use of the above conventional manufacturing method leads to the problem that among MEMS switching elements 90 that have been simultaneously manufactured, a space containing a movable electrode 92 is not sealed completely in some MEMS switching elements 90. In the following, we describe reasons for the problem with reference to FIG. 25 and FIG. 26.
After the above resist pattern is generated as shown in FIG. 25A, wet etching takes place as shown in FIG. 25B. Furthermore, as the etching progresses, the surface of glass wafer is exposed as shown in FIG. 25C.
Now, ideally, as shown in FIG. 25D, the etching may be terminated with all the MEMS switching elements 90 free from side etching. However, as etching rate fluctuates on the surface of glass wafer, as shown in FIG. 25E, side etching occurs in the area where the etching rate is faster than the normal rate.
In the area with a higher etching rate, in particular, an end of the resist is inclined toward (i.e., sags to) the side of the glass wafer as shown in FIG. 25F (i.e., the side of the glass substrate 91a). Hence, supply to the resist of etchant closer to the glass wafer (for instance, in the case that the above metal thin film is gold (Au), potassium iodide solution (Kl) etc.) exceeds that of etchant closer to the resist. Thus, as shown in FIG. 25G, a shape of the metal thin film will be so-called inverted-mesa structure. In other words, area of a contact surface with the glass wafer, with respect to the metal thin film, will be smaller than that of a surface opposed to the contact surface.
Consequently, as shown in FIG. 26, the fritted glass 93 cannot completely flow into a gap G3 that resulted from the inverted-mesa structure. This generates the MEMS switching elements 90 in which the space containing the movable electrode 92 (predetermined space) is not perfectly sealed.
In addition, a similar problem is generated in MEMS devices other than MEMS switching elements 90.